1. Field
The present disclosure relates generally to determining timing in a wireless communication system and, more specifically to methods and apparatus for determining timing in a transceiver of a wireless communication system to set the start of a sampling period.
2. Background
Certain types of communication systems such as those using Orthogonal Frequency Division Multiplexing (OFDM), for example, are highly sensitive to synchronization errors such as timing error and frequency error. In order for these types of systems to work properly, the transceiver and the transmitter have to be synchronized, which includes timing and frequency synchronization. Ideally, synchronization and timing in a transceiver should follow the transmitter. In OFDM systems, for example, timing synchronization, in particular, involves finding the timing of the beginning of each OFDM symbol. Unless the correct timing is known, the transceiver cannot remove cyclic prefixes occurring between symbols at the right timing instant of the symbol and correctly separate individual symbols before computing the Fast Fourier Transformation (FFT) of the sample for demodulating the symbol.
Currently, standard techniques for timing synchronization in wireless communication systems using protocols such as OFDM are not known. Timing synchronization is typically performed on an ad hoc basis. In a channel having 1024 samples, for example, when performing timing synchronization, the start of channel or symbol must be found. In some cases, it is known to look at the rate or differential and then set the start where symbol power starts increasing at some predetermined rate. Some arbitrary back off or offset is then set from that point to ensure that the entire symbol is received in the set time period. Problems with this methodology, in particular, is that timing demodulation might fail if a subsequent symbol appears early during the set sampling period such as in cases where multipath transmission can cause a new symbol to occur in the same timing window. That is, if the subsequent channel appears early, the timing moves because of clock timing errors.
Another known approach for setting symbol timing is to position the symbol approximately in the middle of the sampling window. This is problematic, however, because Intersymbol Interference (ISI) as well as inter-carrier interference (ICI) may occur at either end of the timing window. These two types of interference together can be characterized as “effective interference” (EI). Thus, this approach also may yield timing demodulation errors.
Accordingly, a goal of timing tracking in a system such as an OFDM system is to find the optimum sampling start position of a sampling window for the next OFDM symbol or channel given the current OFDM symbol or channel. The sampling position should be chosen such that intersymbol interference (ISI) as well as intercarrier interference (ICI), which are termed as the “effective interference” (EI), caused by the existing channel profile is suppressed and the signal-to-noise ratio (SNR) is correspondingly enhanced. Sources of EI can be classified into a number of types. The first type is static EI that, given the current channel profile, is a deterministic EI determined by the OFDM symbol structure, e.g., the length of a cyclic prefix. In a dynamic environment, however, channel time variation (new arrival paths that may appear in the future) and system timing error (e.g., sleep timing error) may also introduce EI. This EI can be typified as a dynamic EI that is random in nature and best described by probabilistic models. Another type of EI arises from channel fading, where faded channel taps may also affect the timing decision causing EI.